1. Field of the Invention
The present invention relates to a decoder for decoding coded data, which is preferably applied to a digital control channel receiver of a TDMA cellular system in accordance with the North American Standard of TDMA (Time Division Multiple Access), such as TIA (Telecommunications Industry Association)-TDMA (IS-136).
2. Description of the Background Art
The digital channels of the TDMA cellular system in accordance with the North American Standard include digital control channels (DCCHs hereinbelow) for conveying information required for call control from a base station to mobile stations, and digital traffic channels (abbreviated DTCs hereinbelow) for conveying user's information.
The down link communications from the base station to the mobile stations using the DCCHs is carried out on a superframe basis. The superframe consists of 32 time slots in full-rate transmission, and 16 time slots in half-rate transmission. Information describing the position of a slot in the superframe is called a superframe phase (abbreviated SFP hereinbelow). The SFP consists of an 8-bit code of modulo 32 (although a 5-bit code is enough, the 8-bit code is used for the purpose of simplifying a processing circuit). In the half-rate superframe, only odd- or even-numbered slots are used.
As shown in FIG. 2, the superframe consists of a fast broadcast channel (abbreviated F-BCCH hereinbelow), an extended broadcast channel (abbreviated E-BCCH hereinbelow), a short message service broadcast channel (abbreviated to S-BCCH hereinbelow), reserved slots and Smsch, Pch and Arch channels (abbreviated SPACHs hereinbelow).
The F-BCCH is a channel used for transmitting known information such as structure variables of the DCCHs, parameters needed for access to a system, or the like. Parameters NTH1, NTH2, and DVCC which will be described later are transmitted from the base station to the mobile stations over the F-BCCH. The E-BCCH is used for transmitting information less critical in time than that transmitted over the F-BCCH. The S-BCCH is used for short message broadcast services The SPACHs are used for calling or transmitting orders (PCH), responding to the access from the mobile stations (ARCH), and point-to-point short message services (SMSCH).
Although the number of slots of the F-BCCH, E-BCCH, S-BCCH and SPACHs differ from superframe to superframe of respective frequencies depending on the purpose (chiefly, whether they are used for message services or waiting), the SFPs are assigned in any superframes in such a fashion that they change in ascending order from 00h (h is a hexadecimal notation) to 1Fh in the order of F-BCCH, E-BCCH, S-BCCH, reserved slots, and SPACHs, as shown in FIG. 2.
The SFP of the initial slot of the E-BCCH (called NTH1 hereinbelow), and the SFP of the initial slot of the SPACHs (called NTH2 hereinbelow) are obtained by decoding the F-BCCH followed by analyzing the obtained data.
As shown in FIG. 3A, each slot (consisting of 324 bits) in the superframe consists of a 28-bit synchronizing signal (SYNC), a 12-bit random access control signal (shared control feedback: SCF), 130-bit data (DATA), a 12-bit coded SFP (called CSFP below), 130-bit data (DATA), a 10-bit random access control signal (SCF), and two reserved bits (R).
As mentioned above, the 8-bit SFP is coded into a 12-bit CSFP. More specifically, the SFP is coded into a (12, 8) Hamming code, and the resultant 4-bit parity bits are inverted and added to the SFP to form the CSFP. Accordingly, the mobile station can find the location of the current slot in the superframe by obtaining the SFP by decoding the CSFP.
As shown in FIG. 3B, the data section (DATA) consists of information which is the body of the transmitted data, a CRC which consits of check bits for error detection or error correction of the information, and a tail bit representing the end of the data section.
The methods of calculating the CRC of the data sections at a receiver (mobile station) can be classified into three types A, B and C, depending on the types of channels to which the slot belongs. More specifically, the calculating methods of the CRC are classified into three types A, B and C as shown in FIG. 4, depending on the value of a check code (abbreviated to DVCC hereinbelow) and whether or not the parity bits are inverted when used. The DVCC determines the generator polynomial used for calculating the CRC, and varies depending on the frequency of the superframe. The calculating method A is used for the slots belonging to the F-BCCH, in which the DVCC with a value zero is used, and the parity bits are inverted. The calculating method B is used for the slots belonging to the E-BCCH, S-BCCH and reserved slots, in which the DVCC with a value designated by the base station (BS) is used, and the parity bits are inverted. The method C is used for the slots belonging to the SPACHs, in which the DVCC with a value designated by the base station is used, and the parity bits are not inverted.
The slots of the F-BCCH use the DVCC with a value zero because the value of the DVCC can be obtained for the first time after analyzing the F-BCCH data as in the case of the above-mentioned NTH1 and NTH2.
Thus, any of the methods of calculating the CRC of the received data is used depending upon the channels to which the slots belong. Therefore, the types of the channels (classified in terms of the methods of calculating the CRC) to which the slot belongs must be recognized before performing decoding, detection and correction of the received data in the slot. Here, the types of methods of calculating the CRC are conventionally recognized by obtaining the SFP of the slot by decoding the CSFP, and by comparing the obtained SFP with the parameters NTH1 and NTH2.
There is the possibility in the conventional method, however, that the type of the channel to which the slot belongs cannot be identified, and this will degrade the accuracy of decoding the data. The reason for this is as follows. As described above, the CSFP employs a Hamming code. On the other hand, the data section (DATA) uses convolutional codes. According to the Recommendations to the digital channels of the TDMA systems based on the North American Standard, the free space distance of the convolutional codes is greater than that of the Hamming code, which means that the error correction performance of the convolutional codes is greater than that of the Hamming codes. Thus, there is the possibility that the CSFPs consisting of Hamming codes cannot be correctly decoded, which will lead to errors in decoding, error detection and correction of the received data (DATA). This will cause degradation in the accuracy of decoding the data.
That problem arises not only with the digital control channels of the TDMA cellular system according to the North American Standard, but also with other digital communication systems which switch the error control methods depending on the slot location in the frame.